In the present invention, a substrate made by α-alumina (Al2O3) single crystal (hereinafter, it is called as sapphire) is called as sapphire substrate, and a substrate made by polycrystalline alumina (Al2O3) is called as polycrystalline alumina substrate. The sapphire substrate and the polycrystalline alumina substrate are both called as alumina substrate.
A crystalline layer made of a group III nitride semiconductor such as gallium nitride (GaN), aluminum nitride (AlN) or aluminum gallium nitride (AlGaN) or the like is noted as a functional layer constituting a light emitting device and a power transistor of a light emitting diode or a laser diode or the like which emit a short-wavelength light in the range of blue color to ultraviolet. Further, AlN is also a material which can be expected to be used as a heat dissipation material making use of the high thermal conductivity.
For these crystalline layers, methods are proposed where multi-layered thin-film layers are deposited using vapor deposition methods such as molecular beam epitaxy method or metal organic chemical vapor deposition method or the like on a substrate of such as sapphire or SiC single crystal or the like. However, there is some discrepancy between the lattice constants or the thermal expansion coefficients of the substrate material and the semiconductor crystals, thus, defects or deformation with high density will be introduced during the deposition process. As a result, it will bring decrease in the energy efficiency of the semiconductor element, shortening in the lifetime of the element, inferiority in the property and reduction of yield due to crack.
As a method to solve the problem, substrates with same materials which are excellent in lattice matching are discussed. For example, respect to the crystalline layer of AlGaN containing large amount of Al, different methods are discussed, such as sublimation methods on the substrates of sapphire or SiC single crystal, vapor deposition methods such as hydride vapor deposition method (HVPE) or the like, or methods in which AlN crystal is obtained by flux method. Further, it is desired to deposit a crystalline layer of AlGaN on the independent substrate of AlN single crystal, wherein, the independent substrate of AlN single crystal is obtained by eliminating the substrate of sapphire or the SiC single crystal or the like by milling. Thus, it is required that the AlN crystal grows to a thickness of 100 μm or more preferably. However, due to the growth on different substrates, internal distortions are accumulated; defects, cracks or warping are included. Thereby, different countermeasures are discussed.
As one of the countermeasures, a method of further growing an AlN single crystal on the independent substrate is proposed. By this method, the quality is expected to be improved; but the process is too complex so that the cost is increased. Thus, there is a problem that the utility value in the industry is decreased.
Further, as other methods, Patent Document 1 discloses a method of preparing an AlN layer on the surface of a sapphire substrate by nitriding the surface of a sapphire substrate. In this method, a N2—CO mixed gas is introduced into the heat-treating part disposed with a sapphire substrate and graphite while the composition of the mixed gas is adjusted at the same time. An AlN layer of 5 to 20 nm can be obtained by heat-treating under 1675° C. The mismatching of the crystal lattice between the sapphire substrate and the AlN single crystal film can be eased through a transition layer and thus the threading dislocation can be prevented from occurring which is inevitable in the conventional AlN single crystal film.